The present invention has for its object to provide a method of obtaining by liquid hetero-epitaxy at least one monocrystalline layer of a ternary compound on at least one monocrystalline substrate of a binary composition, in which a solution saturated with the ternary compound is brought into contact with said the substrate, the assembly being raised uniformly to a first temperature, thereafter cooled at a first temperature decreasing rate to a second temperature in such a manner as to effect the resultant epitaxy, said layer having a thickness equal to at least 100 microns.
In accordance with the prior art, this conventional method renders it possible to obtain with a maximum thickness of approximately 100 microns a epitaxy-produced layer. A crucible corresponding to this method is disclosed in Applicants' Patent FR 1600341.
The technique used consists in placing one or a plurality of GaAs substrates in a crucible and covering them by a saturated ternary solution (GaAlAs). The growth thickness is limited because of the fact that the solution becomes locally poor in Al proportionally to the growth and the cooling. Producing transparent diodes on a transparent GaAlAs substrate, which combine a high efficiency with a low power consumption, requires a ternary layer having a thickness of at least 150 microns.
Several recently published articles, more specifically the articles published in "Electronique Industrielle", no. 133, November 1987, pp. 27-28, explicitly state that the technological problems as regards the production of 150 micron thick GaAlAs layer are not resolved. Especially the voltage outlets of light-emitting diodes having a transparent substrate appear to be very important.
It is moreover clear that the methods which are known at present and have for their object to improve the epitaxial growth conditions originate from laboratory techniques, which are not suitable for use in industrial production methods a satisfactory cost.
A known method which improves the growth of layers of a binary composition, specifically of GaAs, has been proposed by J. T. DALY et al in the Journal of Crystal Growth 78 (1986), North Holland pp. 291-302. It consists in producing a permanent laminar circulation of the solution on the substrate in such manner as to ensure that the solution is constantly saturated at the interface with the substrate. This method is complicated and difficult to implement and because of its cost it is more suitable for laboratory activities than for industrial manufacture.
In addition, no evaluation has as yet been made as regards its ultimate efficiency for the growth of ternary layers.
L. E. STONE et al. in "Growth of thick Ga.sub.x Al.sub.1-x As layers by liquid phase epitaxy" (Texas Instruments Inc) have suggested a method to produce thick GaAlAs layers, in which a GaAs substrate is deposited on the surface of a saturated solution, the assembly being placed in an oven which does not only produce a homogeneous temperature of the assembly, but a vertical temperature gradient, which ensures the motion of ternary material in the liquid phase and a continuing growth. The growth is effected at a constant temperature or with not more than a slight temperature decrease in the course of time.
This second prior-art method renders it possible to process the substrates one-by-one, and its is not suitable for either a collective processing of substrates, nor for the successive realization, without complete cooling of the substrates, of several epitaxy layers. In addition, the stabilization of the temperature gradient is comparatively long. This method is therefore not at all suitable for industrial application.
The present invention therefore has for its object to provide an improved method for the manufacture of layers for example made of GaAlAs, by means of hetero-epitaxy on a binary substrate, for example made of GaAs, in such a manner as to obtain greater thicknesses, of the order of 200 microns and more for the GaAlAs.
The invention is based on the idea to utilize the existence of a polycrystalline crust on the bath surface, at the end of a conventional large-thickness epitaxy (approximately 100 microns) effected in a crucible in which the substrate is covered by the solution and, cooling with a temperature decrease between two temperatures. Applicants have found that this crust was of a ternary composition, consequently containing 50% of arsenic, and have put forward the hypethesis that the crust originates from GaAlAs micro-nuclei which form spontaneously during the decrease in temperature and which, being less dense than the solution, start floating on the bath surface, forming a solid mass there.
Now, to obviate the phenomenon that the As solution becomes poor in the region of the crystalline growth interface, efforts have been made to provide a circulation which is adequate to locally enrich the As solution.
Applicants have found that by placing the substrate to be epitaxied into contact with the surface of the solution, which could a prior; be advised against because of precisely the formation of an unwanted crust at the end of the epitaxy operation on this surface, it is possible to obtain much greater epitaxy-produced thicknesses, all the other factors remaining the same, than in the case in which the substrate is covered by the solution. Applicants explain this surprising phenomenon from the fact that these nuclei which are rich in As and flow at the level of the crystallization interface, are dissolved again in the interface layer between the solution poor in As and the solutions enriched in As, which renders it possible to proceed still further proceed with the growth.
The method according to the invention is therefore characterized, in that said contacting the first solution of the ternary compound, for example doped GaAlAs, with the substrate, for example made of doped GaAs and being of the opposite type is realized by placing a lower surface of said substrate into contact with the surface of said solution, in that the first temperature is comprised between 850.degree. and 880.degree. C., in that the second temperature is of the order of 700.degree. C. and in that the first temperature decreasing rate is comprised between 0.6.degree. and 1.2.degree. C./mn.
It should furthermore be noted that from the U.S. Pat. No. 3,762,367 it is known to effect an epitaxial growth by placing a substrate on the successive surfaces of several epitaxial solutions, but the method thus described specifically relates to thin epitaxial layers (not more than 12 microns) with temperature decreases of not more than 20.degree. C. In these conditions and at these temperatures, the phenomenon of a possible formation of a crust at the surface of the solution is not absolutely certain to occur. This phenomenon of a crust forming on the surface of the solution actually occurs when a layer of a great thickness is produced by epitaxy. Consequently, the U.S. Pat. No. 3,762,367 does not have any teachings to overcome the opinion of a person skilled in the art as regards this type of technique, for the realization of layers of a great thickness.
In accordance with a preferred variation, the method includes a final stage in which the epitaxied substrate is placed in contact with the surface of a second saturated solution of the ternary compound, the assembly being cooled at a given second temperature decreasing rate to a third temperature in such manner as to obtain a second monocrystalline layer, more specifically in such manner as to provide a junction between two epitaxied layers and, for example, to finish the manufacturing operations with a second layer of a conventional thickness for forming a light-emitting diode. Thus, it is no longer necessary for the epitaxied substrate to be cooled to ambient temperature between the two epitaxy operations.
The method may include, more specifically in view of the production of light-emitting diodes with a transparent GaAlAs substrate, a stage in which the substrate of the binary composition is removed.
The invention also relates to a crucible for double-bath epitaxy in which the substrates to be epitaxied are placed in a substrate carrier member arranged in a crucible member, and comprising at least one slide which, when displaced, renders it possible to bring the substrates successively into contact with the two epitaxy baths.
Such a crucible is disclosed more specifically in the French Patent Specification FR 1600341, filed by Applicants. In this case the slide is formed by the substrate carrier member itself and the solutions come into contact with the substrates disposed on the bottom of the carrier substrate member slide.
For the above-described reasons, the desired mode of epitaxy is to place the substrates into contact with the surface of the liquid, and, moreover, to effect two epitaxies one after the other, the first one to obtain a transparent substrate, and the second to form a diode therewith, which avoids the necessity of an intermediate cooling of the epitaxied substrate and consequently prevents the disadvantage of any oxidation of Al.
Therefore, the crucible according to the invention is characterized, in that the substrate carrier member is in a fixed translational relation to the crucible member, in that a centre region is present in which a receptacle is formed having a bottom provided with at least one aperture which covers a large portion of the lower surface of the substrates when in position there, in that it has first and second slits arranged in raised edges bounding said centre regions on both sides, in that it includes, arranged above the substrate carrier member a solution carrier member which is in a fixed translational relation with the crucible member and contains a first and a second reservoir intended to receive a first and a second epitaxy solution, respectively, and whose bottom has a third and a fourth slit arranged directly over the first and second slits, in that it comprises, arranged below the substrate carrier member, a first slide which, in a centre region, has a raised edge having an upper surface of a size which is at least equal to that of the first and second slides, and a first and a second portion on both sides of the raised edge, in that it has a second slide which is superposed on the second portion of the first slide, the second slide having a thickness substantially equal to the thickness of said raised edge and having, near an edge which is contiguous to said raised edge, a hole which can accommodate a locking pin which by means of its weight rests on the upper surface of the second slide, the locking pin being provided at a first end of the body of the crucible contiguous to the second reservoir, such that the first slide is capable of translation between a rear position in which it abuts on a second end of the body of the crucible contiguous to the first reservoir and a leading position wherein, on the one hand, the raised edge of the first slide then being in a position in which it does no longer close the first slit, the first solution which has flowed over the first position of the first slide just grazes the lower surface of the substrate and wherein, on the other hand, the locking pin has dropped back by its own weight into the hole of the second slide, which because of the return of the first slide to the rear position causes the first solution to flow back to the first reservoir and the second solution to flow over the second portion of the first slide in such a manner that it grazes the lower surface of the substrate.
In accordance with a preferred embodiment, the aperture in the substrate carrier member has ribs which are spaced apart in such a manner as to receive the edges of the substrate, and the substrates are covered by a plate of the same dimensions as the receptacle.
This renders it possible to ensure that the solution will not contaminate the upper surface of the substrates, and limits its flow.